I have a Dahon Matrix that I use for my winter bike. I'll be putting studded tires on it for ice patches this year (Nokian W106), and it has fenders, rack, lights. Since I don't ride my bike at night, I fold it up and put it on the Metro for the evening commute.

I would like to get a smaller folding bike (SpeedPro or Helios), but it's not in the budget right now. I have not noticed if Schwalb or Nokians make 20" studded tires. A 20" folder with studded tires would be a good winter bike for me, since the Matrix is a bit bulky on the Metro.

My non-folding bikes are steel, but I don't mind the alu Matrix.